The present invention relates to a method for transmitting radio signals and to a receiver for receiving radio signals.
In P. Bureau: xe2x80x9cCompatible digital broadcasting for AM radio bands,xe2x80x9d IBE, March 1999, pages 53 to 54, a digital transmission method based on OFDM is to be used in the short wave, medium wave and long wave range. It is planned to design this xe2x80x9cDigital Radio Mondialexe2x80x9d system (DRM) to be compatible with the radio frequency pattern of the analog short, medium and long wave transmission methods. In so doing, the capability of broadcasting a radio station on several frequencies will be preserved.
With FM/RDS according to ETSI Standard EN 50067 entitled xe2x80x9cSpecification of the Radio Data System (RDS) for VHF/FM sound broadcasting in the frequency range from 87.5 to 108.0 MHz,xe2x80x9d the switch is made to an alternative frequency for a radio station, the identification number of the particular radio station contained in the RDS signal being analyzed after the frequency change.
In contrast, the method and the receiver according to the present invention have an advantage in that the receiver for a radio station selected by the user searches for the transmitter having the best signal quality. The search is screened off from the user so that only the name, for example, of the radio station in question is indicated to the user.
Since the transmitter with the best reception quality is sought, the best reception quality is always made available for the user. Moreover, it is an advantage that the radio station is recognized by the receiver via an identification number which the receiver determines from the radio signals. This ensures an unambiguous assignment of the received radio station by the receiver.
It is advantageous that a change of carrier signal is made without interruption for a radio station that is transmitted on more than one transmission frequency. A user who, for example, is listening to music will not notice any interruption in the music broadcast. This change is advantageously made when the signal quality on the new carrier signal is better than on the previous one. For this purpose, it is advantageous that the determination of the identification number and the determination of the signal quality of the particular radio station is made before the actual change of transmission frequency to ensure that the identical station is being transmitted on this frequency and not that of another broadcasting station. This makes uninterrupted reception of the radio station possible when a carrier signal is switched.
One embodiment according to the present invention provides that the data concerning the carrier signals, the transmission frequencies, in particular, on which the same radio station is broadcast are determined from the radio signals by the receiver so that the receiver for a radio station selected by the user checks these various carrier signals for their signal quality and selects the carrier signal on which the radio station selected by the user is received with the best signal quality.
To provide the receiver time for an uninterrupted check of the other transmission frequencies available, test symbols are provided, from which the receiver determines the identification number of the particular radio station. For these test symbols, a coherent digital modulation method is used, for example, quadrature phase shift keying. This method is distinguished by a high level of interference immunity.
In addition, it is advantageous that by an appropriate modification of the symbol duration of the test symbol without determination of the identification number via correlation methods, it is ensured that the radio station is the same one that the user selected because the different broadcasting stations do not synchronize their broadcasts.
Moreover, it is an advantage that the reference phase and the reference amplitude are determined in a simple manner via pilot carriers for the coherent demodulation of a digital modulation method, for example, QPSK.
The use of a digital modulation method makes it advantageously possible to determine the signal quality via the deviation of the amplitude and the phase of the received radio signal from the expected amplitude and phase. A suitable evaluation of all deviations, such as, for example, the average value, that occur for a test symbol, is taken as a measure of the signal quality.
Another embodiment according to the present invention provides that the test symbols are recognized by a correlation with the protective interval of the test symbols. In doing so, the characteristic of the OFDM symbols, that the protective interval is a copy of the symbol end and is simultaneously a periodic continuation of the symbol, is utilized.
Yet another embodiment according to the present invention provides that state information, such as, for example, frequency and amplification settings, is stored in the receiver for the carrier signal being used at the moment so that the receiver easily returns to the originally used carrier signal with the same settings after having checked the signal quality on the alternative carrier signals.
Moreover, it is an advantage that the symbol pulse setting of the data flow which was determined for the presentation of the radio station selected by the user is also used for the symbol pulse on the alternative frequencies.
One embodiment according to the present invention provides that state information concerning the receiver, such as, for example, amplification and frequency settings, is stored in the receiver for a successfully checked carrier signal on an alternative frequency so that the receiver can start with improved start values for reception in a later check or a continuous change.
In addition, it is an advantage that the receiver according to the present invention makes it possible for only one receive section to be required for the monitoring of the alternative carrier signals. Therefore, the present invention provides a simple and improved product.